old tube radio and antenna impedance
Hello I have two old tube radio aged 1950/60 one with capacitor variable, the second with the inductor variable.
The antenna is just a single wire connected to the rear.
Both cover 500-1620 KHz and 6Mhz-23Mhz
I connected the radio to a single outside horizontal random wire of about 15 metres (as Windom 1923 scheme, 36% off-centre fed) and it works great even if it is less then 1/4 wave. Then I connect the off-centre feed connection point directly to my radio through a single wire of about 10-20 meters running on the terrace floor (depending on the room where the radio are placed). It works great.
My question (i would like to improve the signal)
1) which is normally the input antenna impedance of old tube radio (1950-60') for MW and SW?
2) I tried to use a coax shielded cable (75 ohm) to connect the antenna feed connection point to my radio using just the centre wire without connect the shield to anything at both side (if i connect it to ground or to the radio chassis I loose most of the signal. Is it ok using a coax not to connect the shield to anything? Does it prevent noise at the same?
3) I think it is possible to cut the single wire Windom antenna in two parts and connect them to a coax through a 1:6 balun. I tried but when I connect the shield to the radio chassis I lost most of the signal:.... (I tried also with a dipole without a balun but it doesn't work at the same)... I always have to leave the ending shield unconnected...May be I need a second balun to couple the coax to the high impedance of the radio antenna input? Am I right?
It seems the best is using a single wire.
Any comment or idea to feed and match antique radio antenna input through a double wire?
Hello from Italy!
On the surface, at least, it seems you are attempting to over-complicate your antenna's feedline installation with reference to your receivers' requirements and performance as stated in your posting.
Insofar as you make no mention of problems connected with noise pick-up when you are using the simple and--in your case--seemingly efficient single wire lead-in, I would suggest that you not worry about using coaxial cable...Installing baluns to establish Z (impedance) matches and transformations between unbalanced and balanced loads and characterisitics at both the antenna end and the receiver's end can often degrade performance if the matches are not suitable for the antenna's length, height, and feedpoint location vs. frequencies the receiver is tuned to...Keep in mind, as well, that coaxial cables exhibit rather high losses when you have them installed in a situation where a high SWR (standing wave ratio) is present...The higher your receiving frequency the more these losses become a problem...This condition could be a prime candidate along with any Z mismatches you have present, contributing to the diminished signal levels as you report.
My only suggestion with regard to your present single wire feedline would be to get it up off the floor and have it routed in such a way as to provide reasonable separation from any surrounding objects throughout its length from antenna to receiver which can contribute to loss of signal.
Another consideration for your single wire installation would be to install an antenna tuner (coupler) between the feedline and your receiver's antenna post...This will help you reduce any Z mismatch that exists between your antenna feedline and your radio at the chosen receive frequency...Simple, low cost antenna tuners are available from a number of sources as advertised in the SWL/ham radio literature --or-- you can construct one yourself using a simple coil and vairable cap taken as parts from a defunct radio...There are numerous circuit configurations to experiment with and plans for these are available in numerous locations on the internet.
Although, as I said, it seems the single wire feedline works well for you, if you want to experiment with a dipole or off center fed doublet, such as your Windom, then you may want to consider using parallel open wire feedline...A simple, reasonably low cost option here is either (1)twin-lead (300 ohms characteristic Z) often used for TV and/or FM antenna feedline, or (2) window-line (450 ohms characteristic Z), typically used by amateur radio operators to feed a variety of doublet antenna configurations...Both will have considerably less signal loss than coax providing you keep them dry and separated from objects between the antenna and the receiver.
But what about the coax cable using only the internal wire and take unconnected the shield at both the extremity?
Does it make sense? Is it better than using a single wire?
Hello from Italy!
All that will do for you is to increase the cost of your system.
Most receiver inputs were considered hi impedance inputs and attemts to couple directly to them with low impedance coax is going to result in mismatches and signal loss.
A lot of people don't realize it, but coax is really a poor feedline to use when covering several octaves frequency wise. It has to be terminated in its characteristic impedance or mismatches will develop. A ten ohm mismatch in 73 ohm coax is a lot more, precentagewise than a ten ohm mismatch in a 1000 ohm line.
I have used coax before, but it is not my choice. As soon as I can get the coax to the antenna tuner, then it is open wire, or a single fed wire from there. Usually my total coax run is less than ten feet. From the rig to the low-pass filter, SWR meter, and antenna tuner.
(Connoisseur of the cold 807)
Within my limited knowledge, I see no functional benefit from using a coaxial cable feedline where the shield is left open (without connection) at both ends.
In addition, coaxial cable used in its traditional application can be problematic if moisture finds its way into the insulation between the shield and the center conductor or between the outer covering (the jacket) and the shield...The line's fixed impedance becomes something other than its characteristic impedance and the losses can become very significant over that of its intended design.
Thanks for this kind of information.
But my guess is:
if I don't connect the shield to anything, should it work as a Faraday gage?
I would like to use the unconnected coax only inside the wall of my house (start from when it enters inside)just to prevent the inside RF noise.
So outside the wire is an antenna, inside is just a shielded RF transmission line.
If I don't connect the shield the impedance should not change.
In teory it should be a great idea....
For example if you entirely cover your mobile into alluminium paper it stops to take RF signal...
Hello from Italy!
I got caught in the same trap..., wanting to use coax as a lead-in, but this usually only causes more problems than it solves (especially with older radios).
A good old fashion Marconi antenna usually does the trick for me...
My understanding of a Faraday cage or shield is that is must be a totally enclosed area (for example the interior of a cube having six sides with all six sides connected at their seams and providing continuous electric continuity) to be effective against electro-static transfers.
If the braid of the coax is left open at one or both ends and your intent is to shield the center conductor at lower radio frequencies (in the ranges of interest you stated in your first posting) then I don't think you will realize a practical benefit from the theoretical implications of Faraday shield principles...(This is a guess, insofar as I have no personal experience with such a contrivance).
I think you have stated this understanding yourself, when you said, "For example if you entirely cover your mobile into alluminium paper it stops to take RF signal"... (keyword: "entirely")
As for the other considerations, including the altering of the feedline's characteristic impedance, you stated, "So outside the wire is an antenna, inside is just a shielded RF transmission line.
If I don't connect the shield the impedance should not change"...Here, the characteristic impedance (Z) does change because of the relationship between the conductor (now two at radio frequencies) and surrounding dielectrics (now two insulating materials and air) but this change in characteristic Z is insignificant in terms of the percentage change over a single wire transmission line in free space which is several hundred ohms.
Insofar as your intended use of coax is within your home and thus protected from the effects of weather (moisture), try your idea and compare the results with just your present single wire lead-in...Conduct a simple A=B test at listening frequencies and both ends of your listening spectrum with the same receiver and let us know your results...You may want to expand your testing agenda with the addition of a simple coupler designed for matching unbalanced (single wire) feedlines to your receiver's input...Use the coupler with both lead-in arrangements and compare the results, again at listening frequencies and both ends of your listening spectrum.
Lastly, and very important, have a good earthen ground for your radios...As Martin has said, a good old fashioned Marconi antenna works well and a good ground connection can work in your favor for enhanced signal strength and lower noise in many installations.